Comment by bartbutler

We (ProtonMail) don't provide a server-side IMAP/SMTP interface because we don't want to see your cleartext mail. And if you are doing the encryption and key management yourself locally, then you can literally use any email provider you want. The Bridge sidesteps this issue by exposing a local IMAP/SMTP interface and handling the encryption/decryption locally, but there are very good reasons we built it the way we did.